CODEWITHSUNDEEP
iosmacosgrand-central-dispatchdispatch-queue
starwarrior8809
starwarrior8809

Reputation: 381

How DispatchQueues are implemented under the hood?

I’m curious what DispatchQueue really is under the hood, I tried to google this information but all the documentation is rather abstract and doesn’t provide any real information about the implementation. In my understanding DispatchQueue is some kind of an entity that exists somewhere and is able to store blocks of code and is controlled directly by the kernel(by GCD which is baked into the kernel) which is able to inject those blocks in chosen(by GCD/Kernel) thread. This this the correct vision of DispatchQueue, or I misunderstood something?

Upvotes: 1

Views: 381

Answers (1)

ipmcc
ipmcc

Reputation: 29886

You've misunderstood, at least in some parts. GCD is not "baked into the kernel", it's a library that runs on top of POSIX threads, which are OS-level primitives with kernel support. GCD is simply a set of APIs that make it easier for developers to do work on multiple threads without having to manage the threads themselves.

For what it's worth, you can see the source code for GCD. It's here: https://opensource.apple.com/tarballs/libdispatch/ That said, it's full of micro-optimizations that take advantage of obscure compiler features (branch prediction directives and things like that) and it can often be hard to read and understand, even for experienced systems programmers.

A full-detail explanation of GCD's inner workings is beyond the scope of a StackOverflow answer, but I'll try to cook up a one or two paragraph explanation.

GCD manages some number of POSIX threads behind the scenes that it will use to execute work in the desired way. It also maintains a number of data structures to organize that work, like "queues" which can be thought of as "lists of blocks of work to be done." There are also groups, which allow you to be notified when a list of work items is completed. There are also various IO mechanisms to allow asynchronous IO to be serviced with these work items. It may (or may not) use various kernel services (like threads, kqueues, etc) to manage parts of its workload, but those aren't specific to GCD.

At the end of the day though, there's little or nothing "special" or "blessed" about GCD. In fact, there are multiple ports of GCD to various other operating systems out there, like this one for Linux: http://nickhutchinson.github.io/libdispatch/ which should drive home the point that it's not something specific to the Darwin kernel. Put differently, you could write your own version of GCD from scratch without needing to recompile the kernel.

Upvotes: 5

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